This invention relates to a bicycle with an electric motor and more particularly to an improved electrically assisted, manually powered vehicle.
It has been proposed to extend the usefulness of pedal operated vehicles such as bicycles by incorporating an electric motor that provides a driving assist for the bicycle. Basically, this type of device operates so as to sense the force which the operator applies to the pedals and then apply an added electrical assist to the driven wheel of the vehicle. The amount of electrical assist applied is directly related to the power which the operator places on the pedals.
Of course, it is well known that the force applied by a rider to the pedals will vary cyclically during a single revolution due to the offset of the pedals from the rotational axis. Actually, the pedal force goes from zero (0) at top dead center and increases until the pedals and crank are at a 90.degree. angle and then decreases back to zero (0) for each half revolution. As a result of this operation, the amount of pedal assist during a single crank cycle will vary in direct proportion, with conventional systems, to the operator force sensed at the pedals.
Normally, as has been noted, the assist rate .eta. is a constant and this assist rate is expressed by the following equation:
.eta.=F.sub.M .div.F.sub.L, where F.sub.L equals the manual force and F.sub.M equals the electric motor force.
There are a number of disadvantages with this constant pedal assist relationship.
For example, at low speeds it may be desirable to provide a large electric motor drive force and this results in the election of a large assist rate .eta.. However, if this is used then when the vehicle is being driven at high speed, there will be high electrical energy applied and the speed may become excessive and cause excessive consumption of the battery.
Also, there may be other conditions when it is desired to reduce the assist force under some conditions, but maintain a higher assist force under other conditions.
It is, therefore, a principal object to this invention to provide an electrically assisted, manually powered vehicle such as a bicycle wherein the amount of assist force can be varied.
It is a further object to this invention to provide a vehicle of this type wherein the assist ratio is varied in response to sensed vehicle conditions such as speed.
It is a further object to this invention to provide an improved vehicle of this type wherein the assist ratio can be selectively controlled depending upon operator demand and desires.